A digital transmission link, such as a submarine cable, attenuates data signals which are transmitted along it. Regenerators are devices which are located at appropriate points along the link to reform the digital signals in order to overcome the effects of attenuation. In a digital link each regenerator usually has a clock recovery circuit which generates clock signals from the incoming data signals to provide the necessary timing signals for the various circuits of the regenerator. The clock recovery circuit can include a timing recovery filter or phase locked loop.
It is conventional for each regenerator to have a single timing recovery rate which corresponds to the line rate of the transmission system. This line rate is fixed by the frequency passband characteristics of the timing recovery filter or phase locked loop and cannot be changed once the system is operational.
In some applications there is a need for the transmission systems to transmit at more than one rate. This has been achieved using untimed regenerators but this is not entirely satisfactory.